We purchased stainless steel cable at Home Depot with “locks” that allow the cables to be tightened. Cable was 32 cents/foot...not particularly heavy. Don’t remember strength of cable. We live near beach in FL and can have sudden high winds periodicallly. The shades performed very well.
I wouldn't. Any fabric you cut would unravel. These are pretty sturdy. We have lots of wind and they hold up as they are.
I attach it to the eves with eyehooks
The ones that are perfect 90° are harder to get into the corners, as they need to be an exact match. This can be fluid over your space.